1. Field of the Invention
The present invention relates to a stencil printing machine in which a desired image is heat-sensitively made in a stencil sheet to make a stencil, and an ink is transferred from perforated image areas of the heat-sensitively made stencil to a printing sheet to perform stencil printing, and relates in particular to a technique for improving reproducibility of images.
2. Description of the Related Art
Stencil printing as follows is generally known: a stencil sheet wherein perforated areas are heat-sensitively made to form a desired image is set onto a cylindrical printing drum and then an ink supplied from the inside of the printing drum is transferred through the perforated areas of the stencil sheet to a printing sheet so as to perform desired printing.
More specifically, when in such a type of stencil printing machine a stencil sheet wound in a roll form is transported between a thermal head and a platen roller by a transporting mechanism, the stencil sheet is heat-sensitively perforated therebetween to make a stencil. The thus made stencil is cut into a predetermined length. The cut stencil is wound around a printing drum and attached to the drum. When the made stencil is attached to the printing drum, an ink is supplied from the inside of the drum to the surface thereof with the rotation of the drum. In this way, the ink is forced out from the perforated areas of the stencil.
In synchronization with the above-mentioned operation, a printing sheet fed from a paper feed tray passes through the area between the printing drum and a press roller to which a given pressure is applied, the ink passes through the perforated areas of the made stencil and is transferred to the printing sheet. In this way, a desired image is printed on the printing sheet. The image-printed sheet is then discharged to a sheet discharge tray.
In such stencil printing machines in the prior art, perforations for forming a desired image are heat-sensitively made in a stencil sheet to make a stencil if the stencil sheet in a roll form is always transported at a constant speed.
However, the platen roller arranged opposite the thermal head to be contact-pressed against the head is usually made of an elastic member such as rubber in order to give sufficient transporting force to the stencil sheet. Therefore, the outer diameter of the platen roller changes with a change in surrounding temperature. If the platen roller is controlled at any time so as to have a constant rotating speed, the feed-per-revolution of the stencil sheet transported between the thermal head and the platen roller changes. As a result, the image made by perforating the stencil sheet heat-sensitively expands and contracts, causing a problem that a desired image cannot be made in the stencil sheet.
Such a problem based on the temperature-change can be relieved, for example, by setting a temperature sensor near the platen roller and controlling the driving speed of the platen roller variably on the basis of the temperature-change detected by this temperature sensor.
However, the change in the outer diameter of the platen roller is caused by not only the above-mentioned temperature-change but also the change of the roller itself with the passage of time based on use for a long time. For this reason, only by detecting the temperature-change by the temperature sensor and controlling the driving speed of the platen roller variably, it is impossible to remove an error based on the change with the passage of time and control the feed-per-revolution of the stencil sheet into a constant value.
The platen roller contact-presses against the thermal head at a given grip force. If this grip force changes by the above-mentioned temperature-change or change with the passage of time, the slip amount of the stencil sheet changes. This change in the slip amount also causes the same problem as above.
As described above, stencil printing machines in the prior art cannot relieve the accidental error of the feed-per-revolution of the stencil sheet, based on external factors such as an environmental factor (temperature-change) and a physical factor (change with the passage of time). Thus, an image formed in the stencil sheet expands and contracts. As a result, even if the life of the thermal head does not come to an end, it is impossible to solve the above-mentioned problem of expansion and contraction of the image without exchange of only the platen roller.